Cheatsheet & Examples: watch

Cheatsheet & Examples: watch

The watch command repeatedly executes a program or command and displays its output. This is useful for monitoring the output of a command that changes over time.

Monitor a Command's Output

Example Usage: watch ls -l

What it does: This will repeatedly run the ls -l command and display its output, updating the display every two seconds (the default interval).

Command-line Arguments Explained:

ls -l: The command to be executed. ls lists files, and -l provides a long listing format.

Specifying an Update Interval

Example Usage: watch -n 1.5 df -h

What it does: This will run the df -h command, displaying disk space usage, and refresh the output every 1.5 seconds.

Command-line Arguments Explained:

-n 1.5: Specifies the update interval in seconds.
df -h: The command to be executed. df reports disk space usage, and -h displays it in human-readable format.

Highlighting Changes

Example Usage: watch -d -n 2 ps aux

What it does: This command watches the output of ps aux, which lists all running processes. The -d option will highlight the differences between updates. The output refreshes every two seconds.

Command-line Arguments Explained:

-d: Highlights the differences between updates.
-n 2: Specifies the update interval to 2 seconds.
ps aux: The command to be executed. ps is used to display process information.

Executing a Command and Ignoring Errors

Example Usage: watch --no-title --errexit "ping google.com"

What it does: Runs ping google.com repeatedly and displays its output without a header or error exit.

Command-line Arguments Explained:

--no-title: Suppresses the header that displays the command and update interval.
--errexit: Exits watch if the command being watched exits with a non-zero exit code (an error).
ping google.com: The command to be executed. ping sends network packets to a specified host.

Using a Custom Command to Update the Screen

Example Usage: watch -c cat /proc/loadavg

What it does: This watches the output of cat /proc/loadavg and uses the 'cat' command to render the output to the screen, displaying the system load averages.

Command-line Arguments Explained:

-c: Use command for screen output.
cat /proc/loadavg: The command to be executed. cat displays the contents of a file. /proc/loadavg provides system load averages.